The present invention relates to a conductive lubricant for a fluid dynamic bearing to be assembled into, for example, an HDD (hard disk drive).
Fluid dynamic bearings are increasingly employed in HDD""s in order to provide an increased capacity.
There has conventionally been a fluid dynamic bearing, in which a lubricating oil of an ester-based oil such as diester and polyolester or a polyalkylene glycol oil is interposed between a body of rotation and a bearing member for supporting this body of rotation. This ester-based oil can generate a stable dynamic pressure even when the temperature changes, because it has excellent fluidity at low temperature and a small change in viscosity with respect to temperature change as well as excellent heat resistance.
As an ester-based oil, the conventionally known diester, polyolester or the like is employed. Examples of diester include adipic acid di(2-ethylhexyl) ester (DOA), diisodicyl adipate (DIDA), diisononyl adipate (DINA), di(2-ethyhexyl) azelate (DOZ) and di(2-ethyhexyl) sebacate (DOS). Examples of polyolester include hindered alcohol esters of neopentylglycol (NPG), trimethylolpropane (TMP) and pentaerythritol (PE). These base oils are employed singly or in mixture taking the torque characteristic and operating life of lubrication into consideration.
However, these oils exhibit a very high volume resistivity as apparent from the volume resistivity of each oil indicated in Table 1 below.
If an MR head (magneto-resistive head), which is the other contributor to the increase in capacity, is combined with the fluid dynamic bearing, then the head side (stationary side) and the disk side (rotary side) are completely put in a contactless state in operation. Therefore, in this case, it is required to ground the static electricity occurring at the fluid dynamic bearing, for the protection of the MR head that is vulnerable to static electricity.
Accordingly, the object of the present invention is to provide a conductive lubricant for a fluid dynamic bearing, the conductive lubricant being able to ground the static electricity occurring at the fluid dynamic bearing.
In order to achieve the aforementioned object, the present invention provides a conductive lubricant for a fluid dynamic bearing comprising:
an ester base oil and an antistatic additive having an affinity for the ester base oil, and
the antistatic additive being contained at 0.1 to 5 percent by weight in the conductive lubricant.
According to the present invention, an ester base oil that concurrently has heat resistance, a small quantity of evaporation, low cost and wettability to metal, which are necessary for a fluid dynamic bearing is employed, and an antistatic additive having an affinity for this ester base oil is contained at 0.1 to 5 percent by weight in the conductive lubricant. Therefore, according to the present invention, the conductive lubricant having conductivity by virtue of the antistatic additive contained in the conductive lubricant makes the rotary side and the fixed side of the fluid dynamic bearing electrically conductive. Therefore, the static electricity occurring on the rotary side of the fluid dynamic bearing can be earthed on the fixed side. The antistatic additive has an affinity for the ester base oil, and therefore, the lubrication performance is not impaired.
It is to be noted that the conductivity of oil becomes insufficient if the antistatic additive content is smaller than 0.1 percent by weight and becomes incapable of sufficiently discharging the static electricity. If the antistatic additive content exceeds 5 percent by weight, then the lubrication performance of oil is impaired.
In one embodiment, the antistatic additive is contained at 0.5 to 2 percent by weight in the conductive lubricant.
In the above embodiment, the antistatic additive is contained at 0.5 to 2 percent by weight, and therefore, the conduction performance and lubrication performance of oil can be made compatible at the more desired level.
In one embodiment, an anionic, cationic, amphoteric or nonionic antistatic additive is used as the antistatic additive.